Does delete on a pointer to a subclass call the base class destructor?

Question

The destructor of A will run when its lifetime is over. If you want its memory to be freed and the destructor run, you have to delete it if it was allocated on the heap. If it was allocated on the stack this happens automatically (i.e. when it goes out of scope; see RAII). If it is a member of a class (not a pointer, but a full member), then this will happen when the containing object is destroyed.

class A
{
    char *someHeapMemory;
public:
    A() : someHeapMemory(new char[1000]) {}
    ~A() { delete[] someHeapMemory; }
};

class B
{
    A* APtr;
public:
    B() : APtr(new A()) {}
    ~B() { delete APtr; }
};

class C
{
    A Amember;
public:
    C() : Amember() {}
    ~C() {} // A is freed / destructed automatically.
};

int main()
{
    B* BPtr = new B();
    delete BPtr; // Calls ~B() which calls ~A() 
    C *CPtr = new C();
    delete CPtr;
    B b;
    C c;
} // b and c are freed/destructed automatically

In the above example, every delete and delete[] is needed. And no delete is needed (or indeed able to be used) where I did not use it.

auto_ptr, unique_ptr and shared_ptr etc... are great for making this lifetime management much easier:

class A
{
    shared_array<char> someHeapMemory;
public:
    A() : someHeapMemory(new char[1000]) {}
    ~A() { } // someHeapMemory is delete[]d automatically
};

class B
{
    shared_ptr<A> APtr;
public:
    B() : APtr(new A()) {}
    ~B() {  } // APtr is deleted automatically
};

int main()
{
    shared_ptr<B> BPtr = new B();
} // BPtr is deleted automatically

When you call delete on a pointer allocated by new, the destructor of the object pointed to will be called.

A * p = new A;

delete p;    // A:~A() called for you on obkect pointed to by p

It is named "destructor", not "deconstructor".

Inside the destructor of each class, you have to delete all other member variables that have been allocated with new.

edit: To clarify:

Say you have

struct A {}

class B {
    A *a;
public:
    B () : a (new A) {}
    ~B() { delete a; }
};

class C {
    A *a;
public:
    C () : a (new A) {}        
};

int main () {
    delete new B;
    delete new C;
}

Allocating an instance of B and then deleting is clean, because what B allocates internally will also be deleted in the destructor.

But instances of class C will leak memory, because it allocates an instance of A which it does not release (in this case C does not even have a destructor).

If you have a usual pointer (A*) then the destructor will not be called (and memory for A instance will not be freed either) unless you do delete explicitly in B's destructor. If you want automatic destruction look at smart pointers like auto_ptr.

You should delete A yourself in the destructor of B.

class B
{
public:
    B()
    {
       p = new int[1024];  
    }
    virtual ~B()
    {
        cout<<"B destructor"<<endl;
        //p will not be deleted EVER unless you do it manually.
    }
    int *p;
};


class D : public B
{
public:
    virtual ~D()
    {
        cout<<"D destructor"<<endl;
    }
};

When you do:

B *pD = new D();
delete pD;

The destructor will be called only if your base class has the virtual keyword.

Then if you did not have a virtual destructor only ~B() would be called. But since you have a virtual destructor, first ~D() will be called, then ~B().

No members of B or D allocated on the heap will be deallocated unless you explicitly delete them. And deleting them will call their destructor as well.

You have something like

class B
{
   A * a;
}
B * b = new B;
b->a = new A;

If you then call delete b;, nothing happens to a, and you have a memory leak. Trying to remember to delete b->a; is not a good solution, but there are a couple of others.

B::~B() {delete a;}

This is a destructor for B that will delete a. (If a is 0, that delete does nothing. If a is not 0 but doesn't point to memory from new, you get heap corruption.)

auto_ptr<A> a;
...
b->a.reset(new A);

This way you don't have a as a pointer, but rather an auto_ptr<> (shared_ptr<> will do as well, or other smart pointers), and it is automatically deleted when b is.

Either of these ways works well, and I've used both.